The @fullcalendar/core package was pinned to exact version 6.1.4 while
other @fullcalendar packages used ^6.1.4, causing npm to install newer
versions (6.1.19) that required core@~6.1.19.
This change adds the ^ prefix to @fullcalendar/core version to allow
minor version updates and resolve the ERESOLVE conflict.
Fixes deployment error: ERESOLVE could not resolve peer dependency